1. Field of the Invention
The invention relates to shaft angle encoder interface apparatus, particularly with respect to incremental shaft angle encoders having quadrature channel outputs.
2. Description of the Prior Art
Shaft angle encoders are utilized to measure the angular position of a rotating shaft. For example, the angle of a rotating antenna shaft with respect to a reference point in a radar system is utilized to display radar targets at the appropriate display bearings. Such shaft angle encoders may be of the incremental optical type that provide quardature square waves and a reference pulse which are decoded to indicate the shaft angle. The reference pulse may be utilized to provide an azimuthal reference at, for example, 0.degree. from which shaft rotation is measured. The reference pulse occurs once every 360.degree.. The quadrature square waves relate to the rotation of the shaft with respect to angular position and direction of rotation. The quadrature square waves are generated optically by a rotating code wheel in the optical shaft angle encoder. Two tracks of radial slits on the rotating code wheel generate the quadrature square waves. Each of the tracks is comprised of alternating slits and opaque areas of equal width, one track being displaced 90.degree. with respect to the other track. Light passing through the slits on the rotating code wheel impinge upon photodetectors associated with each of the tracks, thereby generating voltages. Since the code wheel is rotating, the photodetectors receive alternating periods of light and dark thereby generating the quadrature square waves. The reference pulse is generated by a slit on the rotating code wheel at a reference position. The amount of rotation can be determined by counting the number of transitions from zero to a positive voltage and the direction of rotation is decoded by processing the quadrature square waves. Thus, the angular position of the shaft is encoded by the shaft angle encoder as square waves out-of-phase with respect to each other by 90.degree.. These quadrature channel square waves require decoding to provide the angular resolution.
Such shaft angle encoders require interface electronics to convert the quadrature square waves into a digital angle for use in the system. For example, in a radar display system, the digital angle may be utilized by the video system for positioning the radar targets at the proper bearings. The interface logic required to accomplish the decoding conversion from quadrature square wave channels to a digital angle utilizing logic chips, such as gate arrays or discrete logic devices can be extensive, and hence very expensive, particularly with respect to determining the direction of rotation. Alternatively, integrated circuits are available for decoding quadrature channels when interfaced with a microprocessor. A chip of this type is available from the Hewlett Packard Company as the Quardature Decoder/Counter Interface IC denoted as the HCTL-2000. Technical data sheets dated Oct. 19, 1985 for the HCTL-2000 are available from Hewlett Packard and are incorporated herein by reference. The HCTL-2000 decodes quadrature square waves and provides a 12 bit digital angle when interfaced with a microprocessor which provides control signals to the HCTL-2000. The use of the microprocessor is undesirable because of the excessive cost and board real estate required therefor. The microprocessor requires extensive source code assembly language preparation which must be downloaded into program memory. Since the microprocessor must be programmed to effect its control and decoding functions, the cost of the software and the cost of the microprocessor adds an undesired expense to the system.
A further disadvantage of the quadrature channel decoding chip-microprocessor interface is that chips such as the HCTL-2000 have an internal inhibit mode that is automatically enabled during the time that the digital angle is read from the chip output. During enablement of the inhibit mode, the inhibit logic latches the current angle preventing updating from the shaft angle encoder of the chip output angle. Although the digital angle from the shaft angle encoder may change during the inhibit mode, the output angle from the HCTL-2000 is not changed until the inhibit mode is reset. Because the updating of the angle information is limited with such system, the display generally lags some incremental angular error behind the actual shaft position. Although the data is maintained stable during the readout period, the accuracy of the system is adversely affected since the data transfer rate of the system is reduced. It is appreciated, however, that the HCTL-2000 includes internal update buffering for a limited number of clock cycles. Updated data that occurs during the inhibit mode beyond this limited time period is lost.